A switching power converter controller will typically monitor its temperature so that it may lower the switching power converter's output voltage and/or output current if the temperature exceeds a thermal limit. Since the switching power converter is then operating at less than its rated maximum capability, the lowering of the output voltage or the output current (or both) is referred to as thermal de-rating. Generally, existing solutions for thermal de-rating (or thermal scaling) for switching power converters cover only single mode power converters, for example, power converters operating with a fixed output voltage and in constant voltage mode (CV mode). In this case, the power limit (maximum output power for the power converter) is reduced by reducing the output voltage when an over temperature condition is detected in which the state of the power converter exceeds a safe value. Such a thermal de-rating is static and thus independent to the operating conditions of the load device that is being charged.
But a purely CV mode thermal de-rating fails to satisfy the needs of modern power multi-mode adapters or “travel adapters” (TA) that directly charge a portable device through a data cable such as a USB cable or the Apple lightning cable. In general, charging of portable devices over such data cables is subject to a default output voltage level (e.g., 5 V). But in a multi-mode power adapter, there are additional charging modes in which the output voltage carried by the data cable is increased above the default level (e.g., one mode would use 9 V, another mode 12 V, and so on). In addition, note that the direct charging of a portable device battery such as a lithium battery occurs over several phases of constant voltage (CV) and constant current (CC) operation. For example, when a discharged lithium battery is initially charged, a switching power converter such as a flyback converter cannot maintain a constant voltage to such a load in that amount of current required would exceed an initial constant current limit. Charging of the discharged battery thus begins in a constant current mode (CCM) of operation at some maximum current limit (e.g. 4 A). But as the battery is gradually charged, the output voltage over the data cable driving the battery will slowly rise. Eventually, the battery voltage hits a maximum level, whereupon the switching power converter switches to a CCM mode of operation in which the output current limit is decreased. As the battery further charges, the output voltage will again eventually exceed its limit, whereupon the output current is again reduced. Eventually, the direct battery charging ends in a constant voltage mode (CVM) of operation while the output current continues to drop.
But conventional thermal de-rating processes are incompatible with CCM and CVM operation. For example, FIG. 1A illustrates a power adapter (TA) driving an output voltage V and an output current I over a USB cable to a load. The load operates in a constant resistance (CR) mode of operation. Should this power adapter operate in CVM while it thermally de-rates and drives this constant-resistance load, the resulting output voltage and output current waveforms are shown in FIG. 1B. Prior to hitting its thermal maximum, the power adapter maintains the output voltage at 12V with an output current (CC_Limit_1) of 2 A. Should the current exceed the CC limit, the power adapter enters a CC shutdown mode in which the output voltage and output current both drop to zero. In response to the power adapter's temperature exceeding a thermal limit, the power adapter thermally de-rates its constant current limit to 1.5 A (CC_Limit_2). The 6Ω resistance of the load then causes the output voltage to drop to 9V while the output current is kept constant at 1.5 A. Although the constant current limit is not exceeded such the power adapter would not shutdown, the drop of the output voltage from 12V to 9V may cause the load to trigger a fault condition. The load may also operate in constant current mode of operation such as at the 2 A limit shown in FIG. 1B. In that case, the load will continue to draw 2 A despite the change in the power adapter to the 1.5 A constant current limit. The 2 A draw by the load would then cause the power adapter to shutdown such that the output voltage and output current would drop to zero.
Accordingly, there is a need in the art for switching power converters having improved thermal de-rating modes of operation that avoid fault conditions in the load or thermal shutdown of the switching power converter.